Synthetic resin, especially, various types of molded products consisting of materials comprised of engineering plastics also referred to as high performance plastics are widely used for many purposes. For example, they are widely used for housings and chassis for small portable machines with notebook or lap-top style. A weight that is as light as possible is required for housings for said types of small machines.
The easiest method to achieve weight reduction in molded resin products is to reduce the thickness of the film. However, in order to produce a thin-film molded product with a thickness of less than 1 mm, for example, it is necessary to perform the injection at a high pressure so that the molten resin injected for the injection cylinder flows within the narrow cavity at a high speed. When the molten material is injected from the nozzle at a high injection pressure, the pressure of the molten material in the cavity is drastically increased, as a result, the parting of the die is opened. When this happens, it is difficult to produce a molded product with the desired thin-film thickness and design.
It is not practical to provide a closing force greater than the prescribed value to eliminate the above a closing force greater than the prescribed value to eliminate the above-mentioned problems from the standpoint of mechanical structure and cost factors. Therefore, application of compression injection molding, in which the gap between the die cavity is set at a value greater than the prescribed thickness, and closing is done while the molten material is being injected to restore the prescribed cavity gap, is conceivable.
As an example, the compression injection molding method disclosed in Japanese Kokai Patent Application No. Sho 60[1995]-110419 (applicant, Idemitsu Petrochemicals, Ltd.), etc., can be mentioned. However, in a high performance molding that achieves adequate physical properties even when the thickness of the film is low, that is, upon molding of engineering plastics, closing is inhibited due to the internal cavity pressure generated at the final filling stage, especially at the time of production of a thin-film molded product with a film thickness less than 1 mm, and a greater closing force is required; as a result, it is difficult to produce a film with the prescribed thickness.
The objective of the present invention is to eliminate the above-mentioned problems with the conventional method, and to provide a method of producing a thin film capable of producing a molded product consisting of a thin film with a uniform thickness form engineering plastics.
The objective of the invention is to provide a method of producing thin-film molded products that produces a uniform thin-film molded product with highly reproducible properties form high performance plastic resins.